The chemokine CXCL12 (SDF-1) and its cognate receptor CXCR4 are involved in diverse physiological and pathological processes such as HIV infectivity, inflammation, tumorigenesis, stem cell migration, and autoimmune diseases. Although the CXCR4 receptor and its unique ligand SDF-1 have been widely studied, all small molecule modulators of the SDF-1/CXCR4 axis have been antagonists. The lack of available small molecule agonists constitutes a substantial gap in the ability to probe the biology of CXCR4. Using new in silico screening strategies, we have recently discovered the first series of small molecule CXCR4 agonists and have demonstrated their unique behavior in a variety of biological settings. Notably, our small molecules cause internalization of the CXCR4 receptor, a strong chemotactic response, and chemosensitization of tumor cell lines. Development of these small molecule agonists and structurally related antagonists will provide a unique and powerful means to study the function of the CXCR4 receptor and how this relates to disease processes. Acute myeloid leukemia (AML) is a group of myeloid leukemias with a very aggressive and fatal course if left untreated. The bone marrow (BM) microenvironment provides an important protective effect against chemotherapy and disruption of this interaction renders AML cells sensitive to chemotherapy in vitro and in vivo. The SDF-1/CXCR4 axis plays a key role in regulating stem cell mobilization and trafficking and its expression has been shown to negatively correlate to the prognosis of many cancers. Our lead agonist significantly enhances chemosensitivity of multiple leukemic cell lines to several chemotherapies, suggesting that CXCR4 agonists may provide a novel therapeutic approach for the treatment of AML. The overall goal of this project is to optimize small molecule CXCR4 agonist probes and characterize their activity against the CXCR4 receptor and AML in vitro and in vivo. Our unique small molecule CXCR4 agonists and antagonists give us a set of unique molecular tools to understand how CXCR4 receptor pharmacology impacts AML. In Aim 1 we will use rational medicinal chemistry to optimize our lead series for potency and drug-like properties, incorporating in silico design and robust biological testing into an iterative process. We will characterize new CXCR4 modulators in Aim 2 by evaluating their behavior against a number of in vitro systems including calcium mobilization, receptor binding, receptor internalization, chemotaxis, and signaling through various pathways. Aim 3 will evaluate the effects of CXCR4 modulation on AML using leukemia cell lines and primary human tumor cells as well as in vivo using patient-derived xenografts models of AML. The proposed studies will generate new molecular probes to investigate the pharmacology of CXCR4 and understand how this important receptor is involved in AML. These results will provide new insights into AML and potentially open up new avenues for therapeutic development.